Low power control system and associated methods for a water heater with flammable vapor sensor

ABSTRACT

A transistor used as a normally open switch is connected in the millivolt electrical circuit of a fuel-fired natural draft water heater in series with a thermopile device impinged upon by a standing pilot flame, and the solenoid coil portion of the pilot fuel valve. A variable resistance type flammable vapor sensor is connected in a control circuit which is powered by a DC battery and is coupled only to the transistor portion of the millivolt circuit. In the absence of flammable vapor detection by the sensor the control circuit outputs an electrical signal to the transistor that holds it in a closed position to thereby maintain the standing pilot flame. In response to sensor detection of flammable vapor, the control circuit output signal terminates to cause the transistor to return to its normally open switch position, thereby shutting off the pilot flame.

BACKGROUND OF THE INVENTION

The present invention generally relates to fuel-fired heating apparatus and, in a representatively illustrated embodiment thereof, more particularly provides a fuel-fired, natural draft water heater having a specially designed, low power control system which includes a flammable vapor sensor and is operatively connected to a pilot valve millivolt circuit portion of the water heater.

Fuel-fired, natural draft water heaters typically maintain a standing pilot flame which is used to ignite a main burner flame when the control system of the water heater calls for heat to be added to its tank-stored water from the main burner. The pilot burner is supplied with fuel gas through a normally closed valve having an electrically operated solenoid portion used to keep the valve open during the presence of the pilot flame.

The valve solenoid portion is part of what is customarily referred to as a “millivolt” circuit and is wired in series with a thermoelectric device, such as a thermocouple or a multi-thermocouple thermopile structure, which is impinged upon by the pilot flame. Such thermoelectric device operates to convert pilot flame heat to a relatively small amount of electrical current that flows through the millivolt circuit and, via the solenoid, holds the normally closed pilot valve in an open position to maintain the standing pilot flame. Conventionally, the pilot valve is linked to the main burner valve in a manner such that if the pilot valve shuts off the main burner valve automatically does so as well to thereby shut off the water heater combustion process.

In recent years considerable design effort has been expended to provide fuel-fired water heaters with flammable vapor ignition resistance (FVIR) of various sorts in an attempt to prevent the ignition by the water heater of extraneous flammable vapors that may be present (from, for example, spilled gasoline on the floor) adjacent the water heater. One suggested technique to achieve this protective result is to use a flammable vapor sensor which detects flammable vapors adjacent the water heater and terminates or precludes combustion initiation in the water heater combustion chamber. A common type of flammable vapor sensor used in this application is of a chemiresistor type in which the electrical resistance of the sensor increases as a function of the concentration of flammable vapors to which the sensor is exposed. It is this flammable vapor-created sensor resistance increase which is utilized to prevent ignition of such flammable vapors.

One previously proposed technique for using a variable resistance flammable vapor sensor in this application is illustrated and described in published U.S. Patent application 2001/0042564 to Abraham et al, which is hereby incorporated by reference herein, in which a variable resistance flammable sensor 22 is placed in the water heater millivolt circuit in series with the valve solenoid 28 and a thermocouple 36 or thermopile that is position so as to be heated by a standing pilot flame 34. Electrical energization of the solenoid coil 28 with sufficient voltage holds the fuel valve 24 open against the force of a spring 26 urging the valve to its normally closed position.

A disadvantage of this approach of interposing a variable resistance flammable vapor in a fuel-fired water heater millivolt circuit is noted in U.S. published Patent Application 2001/0042564 as being “ . . . it is necessary to increase the internal resistance of the solenoid coil. This can be accomplished by for example the use of higher gauge wire (smaller diameter) and an increase in number of coils in the electromagnet”.

Thus, if it desired to add the flammable vapor ignition protection of a variable resistance type flammable vapor sensor by interposing the sensor in the millivolt circuit in a retrofit application it is necessary to modify or replace the valve solenoid to accommodate the additional electrical resistance (and the corresponding decrease in voltage available to operate the valve solenoid) created by the sensor. This, of course, undesirably adds to the overall installation and materials cost of this retrofit effort. And, of course, a modified solenoid structure would also have to be provided if the flammable vapor sensor was incorporated in the millivolt circuit in the original manufacture of the water heater.

From the foregoing it can readily be seen from the foregoing that it would be desirable to provide a technique for associating a variable resistance flammable vapor sensor with the millivolt circuit of a fuel-fired water heater without the previous necessity of modifying the valve solenoid portion of the millivolt circuit. It is to this goal that the present invention is primarily directed.

SUMMARY OF THE INVENTION

In carrying out principles of the present invention, in accordance with a representative embodiment thereof, a fuel-fired heating apparatus is provided which is illustratively a water heater but could alternatively be another type of fuel-fired heating apparatus such as, for example, a boiler or air heating furnace.

The water heater has a burner, representatively a pilot burner, adapted to receive fuel and combustion air and create a flame therefrom, and a normally closed fuel valve coupled to the burner for supplying fuel thereto when opened. A thermoelectric circuit portion of the water heater has connected in series therein (1) a thermoelectric device positioned to receive heat from the burner flame and responsively generate an electrical voltage, (2) a solenoid structure operative to receive electrical power from the thermoelectric device and responsively hold the normally closed fuel valve in an open position, and (3) a normally open switch device operative to receive an electrical signal and responsively close to thereby permit thermoelectric current flow through the solenoid structure for the duration of said electrical signal.

A low power electrical control circuit, operable by an electrical power source, preferably a long-life DC battery, is coupled to the thermoelectric circuit only via the normally open switch device and is operable to protect the heating apparatus against an undesirable operating condition. The low power electrical control circuit has connected therein a variable resistance sensor operative to detect the predetermined undesirable operating condition which is representatively the presence of a predetermined concentration of extraneous flammable vapors adjacent the heating apparatus. The control circuit, in the absence of detection by the sensor of the undesirable operating condition, outputs an electrical signal to the switch device to maintain it in a closed state, and terminates the electrical signal upon detection by the sensor of the undesirable operating condition to return the switch device to its normally open state which automatically returns the fuel valve to its normally closed position or precludes it from being opened.

By associating the control system and switch with the thermoelectric circuit in this unique manner, which forms a method of the present invention that may be carried out in the initial fabrication of the heating apparatus or later as a retrofit method, the resistance of the sensor, illustratively a flammable vapor sensor, is not incorporated into the thermoelectric circuit. Accordingly, it is not necessary to modify the fuel valve solenoid coil in any manner.

According to various other aspects of the present invention, the normally open switch device is a solid state switch device, preferably a field effect transistor, and the thermoelectric device is preferably a thermopile structure. The low power electrical control circuit preferably includes a multi-resistor bridge section, with the variable resistance sensor forming a resistive portion of the bridge section. Preferably, the control circuit further includes an operational amplifier of the open collector output type and has an input portion operatively connected across the circuit bridge section, and an output portion coupled to the normally open switch device. The control circuit also preferably includes a first electrical lead interconnecting the output portion of the operational amplifier to the normally open switch device, a second electrical lead interconnected between the first electrical lead and the circuit bridge section, and a pull-up resistor connected in the second electrical lead.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view through a representative fuel-fired natural draft water heater embodying principles of the present invention; and

FIG. 2 is a schematic diagram of a millivolt circuit portion of the water heater to which a specially designed low power control system, incorporating a variable resistance flammable vapor sensor, is operatively coupled.

DETAILED DESCRIPTION

As schematically illustrated in FIG. 1, this invention provides a fuel-fired, natural draft water heater 10 which representatively rests on a floor 12 and includes the usual insulated tank 14 that holds a quantity of pressurized water 16 for on-demand delivery to various plumbing fixtures through a tank-mounted outlet fitting 18. Pressurized hot water 16 delivered to such plumbing fixtures is automatically replaced by pressurized cold water delivered to the tank 14 via a tank inlet fitting 20.

Beneath the bottom end of the tank 14 is a combustion chamber 22 in which a main fuel burner 24 and an associated pilot burner 26 are operatively disposed. In a conventional manner, each burner is operative to receive fuel and combustion air which it combusts to create a burner flame. A main fuel supply line 28 is connected to the main burner 24, and a pilot fuel supply line 30 is connected to the pilot burner 26. Normally closed main and pilot fuel supply valves 32,34 are respectively installed in the fuel supply lines 28,30. The pilot valve 34 is linked to the main valve 32 in a conventional manner such that when the pilot valve 34 closes the main valve 32 also closes.

The pilot valve 34 has an electric solenoid coil 36 (see FIG. 2) which circumscribes a movable metal core portion 38 of the pilot valve 26 and, when electrically energized as later described herein, moves the core 38 in a first direction (representatively upwardly as indicated by the arrow 40 in FIG. 2) to open the normally closed pilot valve 34. When coil 36 is de-energized, the solenoid core 38 is spring-driven in the opposite direction 42 to thereby close the pilot fuel supply valve 34, thus also closing the main fuel supply valve 32.

With reference now to FIGS. 1 and 2, according to key aspect of this invention, the water heater 10 is provided with a specially designed low power fuel valve control system 44 which includes a flammable vapor sensor 46. The vapor sensor 46 is positioned near the floor 12 and is operative to sense flammable vapor 48 (such as from spilled gasoline) near the floor 12 adjacent the bottom end of the water heater 10 and responsively cause the pilot valve 34, and thus the main valve 32, to close. The flammable vapor sensor 46 is of the chemiresistor type and has an electrical resistance which increases with increases in the concentration of the flammable vapor 48 to which the sensor 46 is exposed.

Turning now to FIG. 2, the specially designed low power fuel valve control system 44 of the present invention comprises electrical circuitry that includes a millivolt circuit portion 50 and a control circuit portion 52. The millivolt circuit 50 includes a pair of electrical leads 54,46 which are coupled as shown to the pilot valve solenoid coil 36, and to a thermopile device 58 in series with the coil 36. Thermopile device 58, which may alternatively be a thermoelectric device of another suitable type such as a thermocouple, is positioned to be impinged upon by the standing flame 60 of the pilot burner 26 to thereby thermoelectrically generate a small electrical current in the millivolt circuit 50. A conventional ECO (emergency cutoff device) is connected in lead 54 between the coil 36 and the thermopile 58, and a field effect transistor 61 is connected in the lead 54 between the ECO and the coil 36. As will be seen, transistor 61 functions as a normally open solid state electrical switch structure in the millivolt circuit 50. When it is in its normally open mode, the transistor 61 blocks electrical current flow through the solenoid coil 36. When it is in its closed mode the transistor 61 permits electrical current flow through the solenoid coil, and does not add any appreciable additional resistance to the millivolt circuit 50.

It is important to note at this point that the flammable vapor sensor 46 is not directly connected in the millivolt circuit 50. Thus, the valve solenoid coil 36 does not have to be modified in any manner to accommodate the extra electrical resistance of the flammable vapor sensor 46. Instead of being connected in the millivolt circuit 50, the variable resistance flammable vapor sensor 46 is connected in the separate control circuit 52 as will now be described.

Control circuit 52 includes a pair of electrical leads 62,64 between which, from left to right in FIG. 2, electrical leads 66,68,69,70 are connected. A 3.6V long life lithium DC storage battery 72 is connected in lead 66. A 150 kΩ resistor R1 and the flammable vapor sensor 46 (forming a variable resistor R2) are connected in series in the lead 68, and a 150 kΩ resistor R3 and a 49.9 kΩ resistor R4 are connected in series in lead 70. As can be seen, the resistors R1-R4 form a bridge section of the control circuit 52, with the flammable vapor sensor forming a resistive element in one of the legs of such bridge circuit.

An operational amplifier 73, representatively of the open collector output type, is interposed in lead 69 and has an input lead 74 interconnected between its positive input terminal and the lead 70 between the resistors R3 and R4, an input lead 76 interconnected between its negative input terminal and the lead 68 between resistor R1 and the flammable vapor sensor 46, and an output lead 78 connected to gate G of the transistor 61. Thus, the input side of the operational amplifier 73 is connected across the bridge section of the control circuit 52. Additionally, an electrical lead 80 having a 100 kΩ pull-up resistor R5 therein is interconnected between leads 62 and 78 as shown. Pull-up resistor R5 functions to substantially reduce the electrical current outflow from the control circuit 52.

When the flammable vapor sensor 46 is not exposed to flammable vapor 48, its resistance is substantially less than the 49.9 kΩ resistance of resistor R4 so that the total resistance of the R1,R2 circuit leg is similarly substantially less than the total resistance of the R3,R4 circuit leg. Accordingly, under this condition the operational amplifier 73 outputs an electrical signal via lead 78 to the gate “G” of the transistor 61, thereby maintaining the transistor 61 in its closed state and permitting current flow through the transistor 61 to hold the pilot valve 34 open. However, if the flammable vapor sensor 46 is exposed to flammable vapor 48 and its electrical resistance rises to above 49.9 kΩ, the operational amplifier output signal in lead 78 terminates. This causes the transistor 61 to return to its normally open state in which it blocks electrical current flow therethrough, thereby causing the pilot valve 34, and thus the main valve 32, to close.

It should be noted that the control circuit 52 is electrically coupled to the millivolt circuit 50 only by the single electrical lead 78 extending between the operational amplifier 73 and the field effect transistor 61. The electrical power source for the control circuit 52 (the battery 72) is separate from and not coupled to the electrical power source (the thermopile 58). Thus, to retrofit a fuel-fired water heater with the control circuit/transistor structure of the present invention all that is necessary is to connect the transistor 61 in the water heater's millivolt circuit, and then provide the single lead interconnection between the control circuit 52 and the installed transistor. In accordance with principles of the present invention, the control circuit 52, by itself or with the associated transistor 61, may conveniently be provided in module form for original or retrofit installation on an associated water heater.

Instead of the illustrated long-life battery 72 used to power the control circuit 52, other suitable electrical power sources, such as a thermoelectric source or an AC power source converted to DC power, could be alternatively utilized if desired without departing from principles of the present invention. Moreover, in the representatively illustrated control circuit 52, resistors of other suitable values could be utilized, and other circuit components and arrangements could be utilized without departing from principles of the present invention. Similarly, electrical switch structures other than the illustrated field effect transformer 61 could be utilized without departing from principles of the present invention.

Although the present invention has been representatively illustrated as being incorporated in a fuel-fired, natural draft water heater, it will be readily appreciated by those of skill in this particular art that principles of the present invention could alternatively be utilized to advantage in other types of fuel-fired heating apparatus such as, by way of example and not of limitation, fuel-fired boilers and heating furnaces. Additionally, various types of variable resistance type sensors other than a flammable vapor sensor could be incorporated in the control circuit 52, to detect an undesirable operating condition of the associated fuel-fired heating apparatus, could be utilized without departing from principles of the present invention. For example, but not by way of limitation, various other types of gas sensors, or a temperature sensor, could be used.

The foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims. 

1. Fuel-fired heating apparatus comprising: a burner adapted to receive fuel and combustion air and create a flame therefrom; a normally closed fuel valve coupled to said burner for supplying fuel thereto when opened; a thermoelectric circuit having connected in series therein (1) a thermoelectric device positioned to receive heat from said flame and responsively generate an electrical voltage, (2) a solenoid structure operative to receive electrical power from said thermoelectric device and responsively hold said normally closed fuel valve in an open position, and (3) a normally open switch device; and a low power electrical control circuit, operable by an electrical power source and coupled to said thermoelectric circuit only via said normally open switch device, for protecting said heating apparatus against an undesirable operating condition, said low power electrical control circuit having connected therein a variable resistance sensor operative to detect said undesirable operating condition, said control circuit, in the absence of detection by said sensor of said undesirable operating condition, outputting an electrical signal to said switch device to maintain it in a closed state, and terminating said electrical signal upon detection by said sensor of said undesirable operating condition to return said switch device to its normally open state.
 2. The fuel-fired heating apparatus of claim 1 wherein: said fuel-fired heating apparatus is a water heater.
 3. The fuel-fired heating apparatus of claim 1 wherein: said burner is a pilot burner.
 4. The fuel-fired heating apparatus of claim 1 wherein: said thermoelectric device is a thermopile structure.
 5. The fuel-fired heating apparatus of claim 1 wherein: said normally open switch device is a normally open solid state switch device.
 6. The fuel-fired heating apparatus of claim 5 wherein: said normally open solid state switch device is a field effect transistor.
 7. The fuel-fired heating apparatus of claim 1 wherein: said electrical power source is a non-thermoelectric electrical power source.
 8. The fuel-fired heating apparatus of claim 7 wherein: said non-thermoelectric electrical power source is a DC battery.
 9. The fuel-fired heating apparatus of claim 1 wherein: neither of said thermoelectric circuit and said low power electrical control circuit is operative to receive an electrical current flow from the other of said thermoelectric circuit and said low power electrical control circuit.
 10. The fuel-fired heating apparatus of claim 1 wherein: said variable resistance sensor is a flammable vapor sensor.
 11. The fuel-fired heating apparatus of claim 1 wherein: said low power electrical control circuit further includes a multi-resistor bridge section, and said variable resistance sensor forms a resistive portion of said bridge section.
 12. The fuel-fired heating apparatus of claim 11 wherein: said low power electrical control circuit further includes an operational amplifier having an input portion operatively connected across said bridge section, and an output portion coupled to said normally open switch device.
 13. The fuel-fired heating apparatus of claim 12 wherein: said operational amplifier is an open collector output type operational amplifier.
 14. The fuel-fired heating apparatus of claim 13 wherein: said low power electrical control circuit further includes a first electrical lead interconnecting said output portion of said operational amplifier to said normally open switch device, a second electrical lead interconnected between said first electrical lead and said bridge section, and a pull-up resistor connected in said second electrical lead.
 15. A method of inhibiting extraneous flammable vapor ignition by a fuel-fired heating appliance having a burner adapted to receive fuel and combustion air and create a flame therefrom, a normally closed fuel valve coupled to said burner for supplying fuel thereto when opened, a thermoelectric circuit having connected in series therein a thermoelectric device positioned to receive heat from said flame and responsively generate an electrical voltage, and a solenoid structure operative to receive electrical power from said thermoelectric device and responsively hold said normally closed fuel valve in an open position, said method comprising the steps of: installing a normally open switch device in said thermoelectric circuit in series with said thermoelectric device and said solenoid structure; providing a low power electrical control circuit having an electrical power source and a variable resistance type gas sensor incorporated therein, said control circuit having an output portion operative to create and maintain an electrical output signal in the absence of said sensor being exposed to a predetermined concentration of a gas, and to terminate said electrical signal when said sensor is exposed to at least said predetermined concentration of the gas; and coupling said output portion to said thermoelectric circuit, only via said normally open switch device so that the resistance of said sensor is not present in said thermoelectric circuit, in a manner such that said normally open switch is closed during the presence of said electrical signal, and is open in the absence of said electrical signal.
 16. The method of claim 15 wherein: said fuel-fired heating appliance and said thermoelectric circuit are existing apparatus, and said method is performed as a retrofit method in which said low power electrical control circuit and normally open switch device are operatively coupled to the existing thermoelectric circuit.
 17. The method of claim 15 wherein: said installing step is performed using a solid state switch.
 18. The method of claim 15 wherein: said installing step is performed using a field effect transistor.
 19. The method of claim 15 wherein: said providing step includes the step of using a DC battery as said electrical power source.
 20. The method of claim 15 wherein: said providing step is performed using a flammable vapor sensor as said variable resistance type gas sensor.
 21. The method of claim 15 wherein: said providing step includes the step of incorporating said gas sensor as a resistive portion of a multi-resistor bridge section of said low power electrical control circuit.
 22. The method of claim 21 wherein: said providing step includes the step of connecting the output side of an operational amplifier across said bridge section, and said coupling step is performed by electrically connecting the output side of said operational amplifier to said normally open switch device.
 23. The method of claim 22 wherein: said operational amplifier is an open collector output type operational amplifier, said coupling step includes the step of connecting a first electrical lead between said output side of said operational amplifier and said normally open switch device, and said providing step includes the steps of interconnecting a second electrical lead between said first electrical lead and said bridge section, and connecting a pull-up resistor in said second electrical lead. 